1. Field of the Invention
The present invention relates to voltage regulator circuits, and more particularly to circuits for measuring the bidirectional current through a switching device of a switched mode voltage regulator circuit.
2. Description of Related Art
Switched mode voltage regulators (also known as switched mode power converters) are known in the art to convert an available direct current (DC) level voltage to another DC level voltage. A switched mode voltage regulator provides a regulated DC output voltage to a load by selectively storing energy in an output inductor coupled to the load by switching the flow of current into the output inductor. A buck converter is one particular type of switched mode voltage regulator that includes two power switches that are typically provided by MOSFET transistors. The power switches are referred to individually as the high side switch and the low side switch, corresponding to their placement within the buck converter as referenced to the voltage source and ground, respectively. A filter capacitor coupled in parallel with the load reduces ripple of the output current. A pulse width modulation (PWM) control circuit is used to control the gating of the power switches in an alternating manner to control the flow of current in the output inductor. The PWM control circuit uses feedback signals reflecting the output voltage and/or current level to adjust the duty cycle applied to the power switches in response to changing load conditions.
It is known to measure the output current level by sensing the current passing through a resistor coupled to the load. The voltage across the sense resistor is detected using a sense amplifier to produce a signal corresponding to the output current. This type of current sense circuit has the drawback of reducing the efficiency of the voltage regulator by the voltage drop across the sense resistor. Alternatively, it is known to use one of the power switches as a sense resistor and detect the voltage drop across the internal resistance between drain and source of the MOS device (RDSON). This alternative approach overcomes the efficiency reduction caused by a sense resistor. Nevertheless, since the current through the power device is bidirectional, it is often difficult or impractical to measure the bi-directional current.
FIG. 1 shows an exemplary circuit 10 to measure the current IP through an MOS power device 12 having an active area A. A second MOS device 14 having an active area A/k is used to split the load current. Gate driver 16 provides the pulse modulated signal to activate the power device 12 and the second device 14. An operational amplifier 20 has a non-inverting terminal coupled to the source of the power device 12 and an inverting terminal coupled to the source of the second device 14. The operational amplifier 20 includes a feedback resistor 18 coupled between the inverting terminal and output terminal. The operational amplifier 20 maintains the source voltage of the second device 14 at the same level as the power device 12, such that the current through the second device 14 is IP/k. The output terminal of the operational amplifier 20 provides sense voltage Vsense that is proportional to the load current IP. Both directions of current IP can be measured with the circuit, but it should be appreciated that the sense voltage VSense will be negative with respect to the source terminal of the power device 12 for positive load currents IP. This requires an auxiliary negative power supply for the operational amplifier 20, which is in many cases unavailable or costly.
Other known current sense circuits are capable of measuring a bi-directional current through a high side shunt resistor without the need for auxiliary power supplies. But, these known circuits are not suited to measure the current of a MOS power device that is continuously turned on and off. Still other known current sense circuits can measure the current through a power switch, while also suffering from limited linear operating range. These circuits are also undesirable because they require sensing devices that are scaled much larger than necessary to avoid measuring errors.
Thus, it would be advantageous to provide a bidirectional current sensing circuit for a power device that has wide linear operating range, minimal matching requirements, and fast response.